Method and apparatus for x-ray analysis with compensation for an interfering element



Sept. 16, 1969 s. BOYCE ETAL 3,467,824

METHOD AND APPARATUS FOR X-RAY ANALYSIS WITH COMPENSATION FOR AN INTERFERING ELEMENT Filed Sept. 3, 1965 4 Sheets-Sheet 1 F/Gi Sept. 16, 1969 1,5, BOYCE ET AL 3,467,824

METHOD AND APPARATUS FOR X-RAY ANALYSIS WITH COMPENSATION FOR AN INTERFERING ELEMENT Filed Sept. 5, 1965 4 Sheets-Sheet 2 FIG. 2.

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METHOD AND APPARATUS FOR X-RAY ANALYSIS WITH COMPENSATION FOR AN INTERFERING ELEMENT Filed Sept. 5, 1965 4 Sheets-Sheet 4 FIG. 4.

COUNT/N6 RATE (COUNTS/SEC) '3 7QUU- I I I r U 4 8 10 United States Patent 3,467,824 METHOD AND APPARATUS FOR X-RAY ANALYSIS WITH COMPENSATION FOR AN INTERFERING ELEMENT Ian Stanley Boyce, Abiugdon, and Thelma Grace Ahier, Steventon, England, assignors to United Kingdom Atomic Energy Authority, London, England Filed Sept. 3, 1965, Ser. No. 484,960 Claims priority, application Great Britain, Sept. 11, 1964,

37,234/ 64 Int. 'Cl. G01n 23/20 US. Cl. 250-515 12 Claims ABSTRACT OF THE DISCLOSURE A method of X-ray analysis using backscattering geometry for the determination of a wanted element in a sample which also contains an interfering element comprises passing primary X-rays into the sample to obtain backscattered or fluorescent secondary X-rays from the wanted element and fluorescent X-rays from the interfering element. The energy of the primary X-rays is selected so that the X-rays from the interfering element are inefficiently excited but the total intensity of the X-rays is measured using a detector which is more sensitive to the X-rays from the interfering element than it is to the fluorescent X-rays from the wanted element or the backscattered radiation. Desirably a radioisotope source provides the primary X-rays.

The present invention relates to X-ray analysis and is particularly concerned with X-ray analysis using a backscattering geometry and a back-scattering or fluorescence technique.

In X-ray analysis, inaccuracies may be introduced into the analysis by the presence, in the sample being analysed, of an element which has an effect on the X-rays being used for the analysis and/or on any fluorescent X-rays emitted by the wanted element. Such an element is conveniently termed an interfering element, and a common effect produced by such interfering elements is the matrix absorption effect. This effect produces a reduction in the intensity of the back-scattered or required fluorescent X-rays as a result of an increase in the concentration of the interfering element. Since the concentration of the interfering element may vary whilst the concentration of the wanted component remains constant, the intensity of the back-scattered or fluorescent X-rays is liable to variations which are independent of the changes in the concentration of the wanted component. Thus, in determining the ash content of coal by a back-scattering technique, iron is a common interfering element and large variations in the intensity of the back-scattered X-rays may result from variations in iron content rather than variations in the total ash content. Also, in the analysis of copper ores by a fluorescent technique, iron is an interfering element and large variations in the intensity of the fluorescent copper K, X-rays may result from variations in the iron content rather than variations in the copper content. Similar effects are encountered in the analysis of tin in tin ores when iron is again the interfering element or in analysis of a zinc mineral in which both iron and lead are interfering elements.

It will be appreciated that such interference effects may be extremely undesirable since they prevent the concentration of the wanted component being measured with the required degree of accuracy.

If suitable incident X-ray energies are used with a back-scattering technique, however, fluorescent X-rays 3,467,824 Patented Sept. 16, 1969 may be excited in and emitted by the interfering element. The emission of these fluorescent X-rays leads to partial, complete or over-compensation for the interference effect.

In British patent specification 965,303 there is described a method whereby the characteristic fluorescent iron K X-rays excited in the iron are used to compensate for the interference effect of the iron in the determination of the ash content of the coal using an X-ray back-scattering technique. The method described is dependent on the use of incident X-rays having an energy component just above the iron K absorption edge of 7.11 kev. to give eflicient excitation of the fluorescent iron K X-rays. The efficiency of excitation is such that over-compensation results and the use of filters is necessary to obtain exact compensation.

However, using X-rays having low energies of the order of 10 kev. necessitates the use of small particles of less than about 0.04 inch due to the small amount of penetration of the sample by the Xrays. If X-rays of greater energy are used, the sample may contain larger particles and thus the amount of sample preparation is reduced. However, using higher energies results in a decrease in the efficiency of excitation of the fluorescent X-rays of the interfering element, and compensation by the fluorescent X-rays is no longer possible. Thus, in the determination of the ash content of coal, using a source of mean energy of 8 kev. filters are required to obtain exact compensation whilst at an energy of about 14 kev., no filters, other than those inherent in the apparatus, are needed, whilst above this energy, insufficient fluorescent iron K X-rays are excited to give compensation.

A further method of overcoming this effect, chiefly when using the fluorescence technique is to determine the quantity of each interfering element present in the sample and then make allowance for this known concentration of the interfering elements on the intensity of the fluorescent X-rays of the wanted element. This method however is not particularly satisfactory since it involves several measurements and a somewhat laborious calculation and furthermore the wanted element may interfere with the measurement of concentration of the interfering element, e.g. the determination of the iron content of tin ores is interfered with by variations in the tin concentration of the ore.

It is the object of the present invention to provide a new or improved method of X-ray analysis using a backscattering geometry and a back-scattering or fluorescence technique.

Thus according to the present invention, there is provided a method of X-ray analysis using back-scattering geometry for the determination of a wanted component in a sample containing an interfering element, such method comprising passing primary X-rays into the sample whereby secondary X-rays are obtained from the sample in a back-scattered direction together with fluorescent X-rays from the said interfering element, the energy of the primary X-rays being so selected that such fluorescent X-rays are relatively inefficiently excited and the energy of the secondary X-rays is greater than the energy of the fluorescent X-rays, and measuring the total intensity of the secondary X-rays plus the fluorescent X-rays, using a detector which is adapted to be more sensitive to the lower energy fluorescent X-rays than it is to the higher energy secondary X-rays.

The term secondary X-rays is used herein to define back-scattered X-rays, whether such back-scattering is effected by the wanted component or the interfering element, and also fluorescent X-rays, if present, arising from the wanted element. Whether or not such fluorescent 'X-rays will effectively be present will depend on whether or not the energy of the primary X-rays is above or 3 below the X-ray absorption edge of the wanted element, and whether the detector is sensitive to them.

It will be noted that, in general, the backscattering technique may be used to measure the proportions of a wanted component such as ash, whilst the fluorescent technique will measure the proportion of a wanted element such as copper. Although this distinction should be borne in mind, for the sake of convenience the term the wanted element is used inclusively herein.

Moreover the term primary X-rays which is used to describe the radiation passed into the sample should be understood to include the higher energy radiation commonly called 'y-rays.

The present invention also includes apparatus for effecting the said process.

Thus, according to a further aspect of the present invention there is provided apparatus for X-ray analysis using back-scattering geometry, such apparatus comprising a sample holding means to contain a sample of the material being analysed, a source of X-rays positioned to pass primary X-rays into the said sample holding means, such source being selected to emit primary X-rays of an energy affording efficient generation of secondary X-rays from the wanted element and affording relatively inefficient excitation of the characteristic X-rays of any interfering element present in the sample, and a detector for X-rays which is positioned to measure the total intensity of such secondary X-rays and the characteristic X-rays of the interfering element which are emitted in a back-scattered direction, such detector being adapted to be less sensitive to the higher energy secondary X-rays than to the lower energy characteristic X-rays of the interfering element.

It will be appreciated that the energy of the primary X-rays used, and thus the nature of the source, will be dependent on both the element whose concentration is being measured and also on the nature of the interfering elements(s). For a given material, however, the nature of the interfering element(s) will be known and thus ciated that although the fluorescent X-rays of the interfering element will not be excited as efficiently as those of the wanted element, care should be taken that the X-rays from the interfering element are not so inefiiciently excited that compensation for the interelement effect is impossible. Thus, in the analysis of tin in a tin ore containing iron as the interfering element, with a source of energy entirely above that required to excite tin K X-rays (excitation energy 29 kev.), the iron K X-rays are not excited with enough efiiciency for compensation to be possible, even with the use of a thin plastic phosphor as described hereinafter. However, if the source used has a continuous energy spectrum extending above and below 29 kev., then the X-rays of energy below 29 kev. are useful for exciting the desired amount of iron K X-rays. Thus, using a promethium-147/ aluminium bremsstrahlung source, which has a continuous spectrum over the range 10-100 kev., the energy in the range 1029 kev. is effective in giving the desired excitation of the iron K X-rays.

Rather than using a source havinga continuous spectrum, as described however, the source may possess two distinct energy components, one of which is effective in giving the desired excitation of fluorescent X-rays from the Wanted element and the other of which gives suitable excitation of the X-rays of the interfering element. One

such source is americium-241, the radiation from which has a low energy component in the range 11-22 kev. and a high energy component of 60 kev. Using such a source, as with a continuous spectrum source, the detector used would be adjusted to have a greater sensitivity to incident X-rays of one energy than to the energy of the other incident X-ray.

Any detector whose energy response can be controlled suitably may be used and plastic phosphor scintillation counter detectors are presently preferred. The efficiency of the plastic phosphor is dependent on the thickness of the phosphor and also on the X-ray energy as shown in Table I for a plastic phosphor type NE 102 A manufactured by Nuclear Enterprises Ltd.

TABLE I Phosphor thickness (10- ins.)

X-ray energy (kev.) 2 15 100 5.9, percent 5 12 23 32 40 72 92 22.2, percent 0.15 0.5 0.9 1. 4 1. 8 4. 4 8.5 88, percent 0.1 0.2 0. 4 0. 6 0.8 2. 0 4. 0 Efiicieucy at 5. ke ./Efli ncy at 22.2 kev 25 24 25 23 22 17 11 the optimum energy of the X-rays from the source will be dependent only on the nature of the material being analysed.

More specifically, the process of the present invention may be used, adopting a back-scattering technique, for determining the ash content of coal by the method of the present invention, in which case an X-ray source giving X-rays of energy in the range between about 14 and 22 kev. should be used. Conveniently, the X-ray source would be a radioisotope source and for the analysis of coal according to the present invention a suitable source would be the radioisotope cadmium-109 which emits X-rays of energy 22 kev. together with a small proportion of 88 kev. X-rays. Using the method of the present invention for ash-in-coal determinations, particles of size 0.5 inch may be used as compared with particle sizes of 0.04 inch when using the method of Patent No. 965,303.

In using the fluorescence technique, it should be appre- Table I also shows the ratio of efficiency for counting 5.9 kev. X-rays to efiiciency for counting 22.2 kev. X-rays. It will be seen that the ratio is effectively at its maximum value with a phosphor thickness of 0.020 inch.

The phosphor may be coupled directly to the photocathode for a photomultiplier but in such as case it may be found that direct excitation of the photocathode is resulting from X-rays passing undetected through the phosphor. In such a case the efiiciency for 22 kev. may be greater by direct excitation than by the 0.020 inch plastic phosphor as shown in Table II.

TABLE II X-ray energy (kev.)

Percent efiiciency of detection by direct excitation of the photocathode In this case, therefore, the ratio of counting efiiciency is reduced from 22 to 7.5.

This effect of direct excitation may be overcome by the use of a light guide between the plastic phosphor and the photomultiplier. The purpose of such a light guide is to absorb the 22 kev. and 88 kev. X-rays efficiently whilst at the same time transmitting, with high efliciency, the scintillations from the phosphor to the photomultiplier photocathode.

The present invention relies on the fact that as the concentration of the interfering element increases the intensity of the fluorescent X-rays emitted by the interfering element increases whilst that of the secondary X-rays emitted by the wanted element decreases due to the matrix absorption effect. Using a detector which is more sensitive to the fluorescent X-rays from the interfering element than to the secondary X-rays from the Wanted element, the increase in the one may be made to compensate for the decrease in the other. It should be appreciated that, using the fluorescence technique, the detector measures not only the fluorescent X-rays from the wanted element but also any back-scattered X-rays and that the back-scattered X-rays are also affected by the variations in the content of the interfering element. Thus, the fluorescent X-rays from the interfering element must be intense enough to overcome both the decrease in backscattered intensity and that in the intensity of the fluorescent X-rays of the wanted element.

It will be realised that the sample used must be of at least the saturation thickness for back-scattering or exciting fluorescence as the case may be. This saturation thickness, which can easily be determined, will depend inter alia on the nature of the source used and also the particular material being analysed.

In order that the present invention may more readily be understood, one embodiment of apparatus for effecting the same will now be described by way of example, reference being made to the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic representation of apparatus for the analysis in accordance with the present invention;

FIGURE 2 shows calibration curves for the analysis of ash in coal using a plastic phosphor with and without a light guide, and using the back-scattering technique;

FIGURE 3 shows, for comparison, calibration curves using a promethium-l47/ aluminum bremsstrahlung source and a sodium iodide crystal detector in the analysis of tin ores using a fluorescence technique; and

FIGURE 4 shows the calibration curve obtained with the apparatus of FIGURE 1 in the analysis of tin.

Referring now to the drawings, and in particular to FIGURE 1 as used in the analysis of ash in coal using a back-scattering technique, X-rays are obtained from a cadmium-109 source 1 in a gold source holder 2. The source 1 gives X-rays of energy 22 kev. with a small amount of 88 kev. X-rays; and is placed below a sample holder 3 containing a sample 4 of coal to [be analysed. The sample 4 consists of particles of coal of size up to 0.5 inch and has a thickness in excess of that required to cause saturation backscattering, i.e. in excess of three inches (5.3 grn./cm.

The sample holder is shown diagrammatically and in practice may be a simple cell, a moving belt, a flow cell or the like.

The X-ray detection apparatus consists essentially of a low-noise photomultiplier 5 and a phosphor 6 which are separated by a light guide 7. The light guide 7 consists of a 4 inch long by 2 inch diameter column of Perspex (polymethylmethacrylate) which provides optical coupling between the photomultiplier 5 and the phosphor 6 and acts both as a light guide and an X-ray attenuator as hereinbefore explained. The phosphor 6 consists of a 0.020 inch thick plastic phosphor type NE 102A. The phosphor 6 is covered by a detector window 8 of 0.0005 inch thick aluminised Mylar, the source 1 and source holder 2 resting on the window 8. The assembly of window 8, phosphor 6 and light guide 7 are retained in position by a tubular member 9 which extends along the length of the light guide 7 and around the end of the photomultiplier 5.

It should be appreciated that materials other than polymethylmethacrylate may be used for the light guide 7, for example glass or quartz, and that using a material of greater density and atomic number the length of the light guide may be decreased whilst still giving the desired attenuation of the 22 kev. and 88 kev. X-rays.

Using the apparatus as shown, 22 kev. X-rays from the source 1 pass into the coal sample 4 where backscattering occurs together with excitation of the iron K X-rays of energy 6.4 kev. The backscattered X-rays and the fluorescent X-rays cause scintillation of the phosphor 6 and these scintillations are detected by the photomultiplier 5, the 22 kev. backscattered X-rays passing through the phosphor 6 being attenuated by the light guide 7. The rate of counting of the photomultipler 5 may be compared with a calibration curve to give the ash content of the coal sample independent of the iron content or, if desired, the photomultiplier output could be calibrated to give the ash content directly.

In FIGURE 2, curve A is obtained using the phosphor 6 alone with no light guide and a coal sample containing no iron and varying amounts of ash. Curve B is similar to curve A for coal samples containing 20% by weight of ferric oxide (Fe O in the ash. Curve C is obtained using the apparatus of FIGURE 1 with samples of coal with 0 and 20% of ferric oxide in the ash.

Thus, the introduction of the light guide makes compensation for the iron content in the ash possible in coalash analysis since the light guide attenuates the backscattered X-rays (energy 20-22 kev.) and thus prevents these X-rays causing direct excitation of the photocathode of the photomultiplier. It should however be appreciated that the use of a light guide may prove unnecessary in extending the present technique to the analysis of materials other than ooal.

Thus the apparatus of FIGURE 1 may be used, without the light guide 7 for the analysis of tin ores containing iron by a fluorescence technique.

In this case, X-rays having a continuous energy spectrum in the range 10-100 kev. are obtained from a promethium 147/ aluminum bremsstrahlung source 1 in the gold source holder 2. The source 1 is placed below the sample holder 3 containing a sample 4 of a tin ore which is to be analysed. The sample 4 is of at least the saturation thickness for tin fluorescence, i.e. at least 0.5 inch (2.5 gms./cm.

The X-ray detection apparatus consists solely of the low-noise photomultiplier 5 and the phosphor 6, the phosphor 6 consisting of 0.020 inch thick plastic phosphor type NE 102A. The phosphor 6 is covered by a window 8 of 0.003 inch thick aluminum which acts as a filter for fine adjustment of the iron K detected intensity. This arrangement gives sufficient discrimination in favour of the iron K X-rays to provide adequate compensation for the Variations in the iron content of the ore.

Using the apparatus as shown, X-rays from the source 1 pass into the sample 4 and excite fluorescence in the tin and iron present inthe sample. The X-ray absorption edge of tin is at 29.2 kev., and thus X-rays of lower energy than 29.2 kev. are effective only in exciting the iron fluorescent X-rays whilst X-rays of energy greater than 29.2 kev. are effective to excite primarily the tin fluorescent X-ray since at these energies the efliciency of excitation of the iron X-rays is low. Some back-scattering of the incident X-rays also occurs. The fluorescent and back scattered X-rays pass through the window 8 which efiects some filtration of these X-rays which then cause scintillation of the phosphor 6. The scintillations are detected by the photomultiplier 5. The rate of counting of the photomultiplier 5 may be compared with a calibration curve to give the tin content of the ore independent of the iron content, or, if desired, the photomultiplier output could be calibrated to give the tin content directly.

In FIGURE 3, curves D and E show the total counting rate (tin K X-rays plus back-scattered X-rays plus iron K X-rays) using a sodium iodide crystal detector, which is approximately equally eificient for detecting all the energies incident thereon in this experiment and ores of varying tin content having iron contents of O and 5% by Weight respectively. If the exciting source is promethium 147/aluminum and a crystal detector is used the efficiency of detection of the iron X-rays is high, but the excitation efiiciency of the iron X-rays is low and thus the iron X-rays do not give compensation for the variation in iron content of the ore. Curves F and G of FIGURE 3 show the counting rate of tin K X-rays only for various tin contents and iron contents of and by weight respectively. The tin X-rays were isolated using balanced filters of silver and palladium. The curves F and G indicate clearly the effect of the iron present in the ore and it .will be clear that accurate determinations of the tin content are not possible using a pair of balanced filters.

In FIGURE 4, curve H is obtained using the apparatus of FIGURE 1 with tin ores containing 0 and 5% by weight of iron. From the curve H therefore, it is possible to determine the tin content of the ore merely by measuring the counting rate obtained using the ore in the apparatus of FIGURE 1. Thus, the use of the plastic phosphor and photomultiplier gives compensation for the ef* fects of the iron content in the ore.

It will be appreciated that the method and apparatus of the present invention as described for the analysis of tin ore, may be adapted for the analysis of other ores containing interfering elements which give matrix absorption effects.

Although this modified apparatus is satisfactory for the analysis of tin ores, it may in other cases be found that such apparatus does not provide complete compensation for the interfering element due to some of the higher energy X-rays from the wanted element passing through the plastic phosphor undetected and giving direct excitation of the photocathode of the photomultiplier. In such a case the apparatus may be modified by the reintroduction of the light guide between the phosphor and the photomultiplier in order to attain the desired compensation.

We claim:

1. A method of X-ray analysis using back-scattering geometry for the determination of a wanted element in a sample containing an interfering element, such method comprising passing primary X-rays into the sample thereby to obtain secondary X-rays from the sample in a back-scattered direction together with fluorescent X- rays from the said interfering element, the energy of said primary X-rays being so selected that such fluorescent X-rays are relatively inefliciently excited and the energy of the secondary X-rays is greater than the energy of the fluorescent X-rays, and measuring the total intensity of the secondary X-rays plus the fluorescent X-rays, using a detector which is more sensitive to the lower energy fluorescent X-rays than it is to the higher energy secondary X-rays.

2. The method of claim 1, including the step of attenuating the higher energy secondary X-rays prior to measurement.

3. The method of claim 1, including the step of preferentially attenuating the lower energy fluorescent X-rays from the interfering element prior to measurement.

4. The method of claim 1, including the step of obtaining the primary X-rays from a radioisotope source.

5. The method of claim 4, and for the analysis of ash in coal wherein the source is cadmium-109.

6. The method of claim 4 and for the analysis of tin wherein the source is a promethium-147/aluminum bremsstrahlung source.

7. The method of claim 4 and for the analysis of tin wherein the source is americium-241.

8. Apparatus for X-ray analysis to determine the proportions of a wanted element in a sample using backscattering geometry, such apparatus comprising a sample holding means to contain the sample of the material being analysed, such sample also containing an interfering element; a source of X-rays positioned to pass primary X-rays into the said sample holding means of an energy affording efficient generation of secondary X-rays from the wanted element and affording relatively inefficient excitation of the characteristic X-rays of the interfering element; and a detector for X-rays positioned to measure the total intensity of such secondary X-rays and the characteristic X-rays of the interfering element which are emited in a back-scattered direction, such detector being less sensitive to the higher energy secondary X-rays than to the lower energy characteristic X-rays of the interfering element.

9. The apparatus of claim 8, wherein said source is a radioisotope source.

10. The apparatus of claim 9, wherein said source has a continuous spectrum only part of which is sufliciently energetic to excite fluorescent X-ray in the wanted element.

11. The apparatus of claim 9, wherein said source has two distinct energy components, respectively effective to excite fluorescent X-rays in the wanted and interfering elements.

12. The apparatus of claim 8, including a filter preferentially to attenuate the characteristic X-rays from the interfering element.

References Cited UNITED STATES PATENTS 2,923,824 2/1960 Martin et a1. 250-71.5 3,270,200 8/1966 Rhodes 25051.5 X

FOREIGN PATENTS 768,534 2/1957 Great Britain.

OTHER REFERENCES The Encyclopedia of X-rays and Gamma Rays, edited by G. L. Clark, published by Reinhold Publishing Corp., New York, 1963, pp. 257, 258, 534 and 535.

WILLIAM F. LINDQUIST, Primary Examiner 

